Ink jet printing processes that employ inks that are solid at room temperature and liquid at elevated temperatures are known. For example, U.S. Pat. No. 4,490,731, the disclosure of which is incorporated herein by reference in its entirety, discloses an apparatus for dispensing solid inks for printing on a substrate such as paper. The ink vehicle is chosen to have a melting point above room temperature so that the ink, which is melted in the apparatus, will not be subject to evaporation or spillage during periods of nonprinting. The vehicle selected possesses a low critical temperature to permit the use of the solid ink in a thermal ink jet printer. In thermal, or hot-melt, ink jet printing processes employing these phase change inks, the solid ink is melted by a heater in the printing apparatus and used as a liquid in a manner similar to that of conventional piezoelectric or thermal ink jet printing. Upon contact with the printing substrate, the molten ink solidifies rapidly, enabling the dye to remain on the surface instead of being carried into the paper by capillary action, thereby enabling higher print density than is generally obtained with liquid inks. After the phase change ink is applied to the substrate, freezing on the substrate resolidifies the ink.
In general, phase change or “hot melt” ink compositions are in the solid phase at ambient temperature, but exist in the liquid phase at the elevated operating temperature of an ink jet printing device. At the jet operating temperature, droplets of liquid ink are ejected from the printing device and, when the ink droplets contact the surface of the recording substrate, either directly or via an intermediate heated transfer belt or drum, they quickly solidify to form a predetermined pattern of solidified ink drops. Phase change inks have also been used in other printing technologies, such as gravure printing, as disclosed in, for example, U.S. Pat. No. 5,496,879 and German Patent Publications DE 4205636AL and DE 4205713AL, the disclosures of each of which are incorporated herein by reference in their entirety. Phase change inks have also been used for applications such as postal marking and industrial marking and labeling.
Phase change inks are desirable for ink jet printers because they remain in a solid phase at room temperature during shipping, long term storage, and the like. In addition, the problems associated with nozzle clogging as a result of ink evaporation with liquid ink jet inks are largely eliminated, thereby improving the reliability of the ink jet printing. Further, in phase change ink jet printers wherein the ink droplets are applied directly onto the final recording substrate (for example, paper, transparency material, and the like), the droplets solidify immediately upon contact with the substrate, so that migration of ink along the printing medium is prevented and dot quality is improved.
U.S. Pat. Nos. 5,531,817 and 5,476,540, the disclosures of which are incorporated herein by reference in their entirety, disclose the control of color bleed (the invasion of one color into another on the surface of the print medium) using ink-jet inks by employing either high molecular weight polymers that exhibit a reversible gelling nature with heat or certain amine oxide surfactants that undergo sol-gel transitions.
U.S. Pat. No. 5,554,212, the disclosure of which is incorporated herein by reference in its entirety, discloses an aqueous phase change ink composition that contains a water dispersible sulfonated polyester gloss agent and a hyperthermogelling component that causes gellation at temperatures at or above its thermo-inversion point or at high concentrations.
While hot melt ink compositions are used successfully, a need remains for phase change ink compositions that are suitable for hot melt ink jet printing processes, such as piezoelectric ink jet printing processes and the like. There is still a need for ink compositions that can be processed at lower temperatures and with lower energy consumption, and a need for inks that have improved robustness and printing latitude. There is also a need for ink compositions that have improved jetting reliability and latitude with respect to meeting both the jetting and transfuse requirements of curable aqueous and non-aqueous inks. In addition, a need remains for phase change ink compositions that exhibit desirably low viscosity values at jetting temperatures. Additionally, a need remains for phase change ink compositions that generate images with improved look and feel characteristics. Additionally, there is a need for phase change ink compositions that generate images with improved hardness and toughness characteristics. A need also remains for phase change ink compositions that are suitable for high speed printing, thereby enabling transaction and production printing applications. In addition, there remains a need for curable ink compositions for piezoelectric ink jet printing that produce a stable image that can be transferred to a substrate without cracking and hardened upon cure.